The present invention relates to a clinker-type hydraulic binder obtained by burning comprising a magnesia spinel mineralogical phase and at least one calcium aluminate mineralogical phase, with a lime content of less than 15% of the binder by dry weight. It also relates to the use and a method of making such a binder.
Ladle steel metallurgy has developed these last years up to become a key point in the steel-manufacturing process. A ladle is a real chemical reactor with internal temperatures adapted to reach 1700xc2x0 C. and being able to contain up to 300 tonnes of melted material. Conventionally used refractory concretes (first shaped, and then more and more monolithic) in steel ladles are not satisfactory any more and the performances thereof in such a field have to be improved.
In particular, the steel ladles contain wear linings in contact with steel and slag and more particularly exposed to slag infiltration and corrosion. Such wear linings should be able to best resist such aggressions.
More particularly there is an interest for monolithic concretes with low lime content (lower than 2.5% by dry weight in concrete) so-called LCC concretes (Low Cement Concrete), and with very low lime content (lower than 1% by dry weight in concrete) so-called ULCC concretes (Ultra Low Cement Concrete). The low lime content of such concretes is advantageous for obtaining a high refractoriness required for applications with steel ladles.
The Applicant has described in publication UNITECR""97, vol. III, pp. 1347-1354 (1997) of N. Blunt, C. Revais and M. Vialle entitled xe2x80x9cAdditives in calcium aluminate cement containing castablesxe2x80x9d, a study over castable monolithic refractory concrete types based on a blend of aluminous cement and magnesian spinel, particularly with a low lime content. The magnesian spinel and calcium aluminates contained in the aluminous cement have therein functions of refractory component et hydraulic component.
The concretes being described in such a publication bring out difficulties to reach a satisfactory rheology and an easy implementation.
Other solutions have been proposed to make refractory concretes through a clinker based on magnesian spinel and calcium aluminates.
Thus, Patent FR-1,575,633 discloses an aluminous refractory cement produced from a blend of 30 to 50% dolomite and 50 to 70% calcined alumina by burning up to clinkerization or melting.
FR-2,043,678 is an addition certificate application attached to FR-1,575,633, which described an aluminous refractory cement based on magnesian spinel and calcium aluminates obtained from a blend of dolomite and calcined bauxite or calcined alumina, lime and magnesia, through burning up to clinkerization or melting.
Japanese Patent Application JP-8-198649 is per se relative to a composition of refractory cement or concrete based on a calcium aluminate material prepared from lime, alumina and magnesia, through melting and/or calcination.
The compositions of the three above-mentioned last documents have the inconvenient that they are not sufficiently well adapted for refractory applications in steel ladles, particularly for producing LCC or ULCC concretes able to resist slag infiltration in steel ladles and resulting corrosion.
The invention relates to a clinker-type hydraulic binder obtained by burning particularly adapted for producing steel ladles and having a very high resistance to slag infiltration and corrosion compared particularly to the known binders.
The binder according to the invention allows to produce refractory monolithic concretes LCC or ULCC based on magnesian spinel, making possible an implementation with very satisfactory reactivity (setting time) and rheology (fluidity, castability).
The invention also relates to the use of such a binder for making a refractory concrete.
It has also as an object a process for making such a binder, making possible an easy implementation from currently available raw materials and advantageously at a low burning temperature (lower than 1800xc2x0 C.).
Other advantages associated with the binder according to the invention, besides the refractoriness and the resistance to slag infiltration and corrosion, include the following ones:
cancellation of the 12CaO.7Al2O3 (so indicated C12A7), except possibly in a strongly underburnt clinker and only on a temporary basis, such phase being able to bring about stiffening difficulties in concrete formulations;
binder microstructure being advantageous for the milling thereof so as to reach high granular fineness, so improving the corrosion resistance;
very low content in free residual magnesia, i.e. non combined in magnesian spinel, so as to be able to prevent the generation of cracks due to free magnesia in brucite during the implementation step for the refractory concrete produced from the binder.
Thus, an object of the invention is a clinker-type hydraulic binder obtained through burning comprising:
a magnesian spinel mineralogical phase, and
at least a calcium aluminate mineralogical phase with a lime content of less than 15% of the binder by dry weight.
According to the invention, the magnesian spinel comprises between 68% and 81% of the binder by dry weight.
Surprisingly such high proportions of the magnesian spinel allow to obtain the above-mentioned advantages, in particular good corrosion resistance properties.
By contrast, the known clinker-type binders comprising magnesian spinel and calcium aluminates and with a lime content of less than 15% have substantially lower magnesian spinel contents. In particular, FR-1,575,633 and FR-2,043,678 disclose proportions comprised between 25 and 45% magnesian spinel.
JP-8-198649 relates as per se to a binder with a lime content comprised between 15% and 30%, thus inappropriate for making LCC or ULCC concretes.
The term xe2x80x9cclinker-type binderxe2x80x9d means not only the proper clinker, thus the product before milling, but also the clinker being milled.
Such a clinker may be produced either at high temperature (higher than 1800xc2x0C.) by melting, for example in an electric oven, or advantageously through calcination (sintering) at low temperature (lower than 1800xc2x0 C.).
Preferably, the binder is used for making concrete to which it gives magnesian spinel fine particles. The concrete formulation is then advantageously supplemented by fine reactive aluminas and by large magnesia spinel, as well as other granulates.
The high proportion of magnesian spinel in the binder allows to supply the total fine spinel of the concrete while preventing the problems encountered with a direct mixture of aluminous cement and magnesian spinel, as in UNITECR""97 above-mentioned. Moreover, the so-made concrete may have a low or very low lime content.
Preferably, the calcium aluminates are all under crystallized form.
More precisely, it is advantageous that the calcium aluminates should be essentially made of CA and CA2, with CA being CaO and A being Al2O3.
Such a binder composition, with a pattern MAxe2x80x94CAxe2x80x94CA2 (with Mxe2x95x90MgO), leads to such a surprising and advantageously consequence that the presence of C12A7 is prevented, such a phase being adapted to lead to a stiffening cement.
Advantageously the calcium aluminates CA and CA2 comprise between 19% and 32% of the binder by dry weight. More particularly it is strongly interesting the binder should comprise by dry weight of the binder:
71xc2x12% MA (magnesian spinel)
18xc2x12% CA, and
11xc2x12% CA2.
Such a composition is in thermodynamic equilibrium in the CaOxe2x80x94MgOxe2x80x94Al2O3 system, so that C12A7 cannot be present in this combination, except possibly in a strongly underburnt clinker and on a temporary basis.
In an alternative embodiment, the calcium aluminates are present under an amorphous form, particularly under a vitreous form.
Preferably, the binder is quasi-free from free residual MgO at least as it can be observed on X-ray diffraction spectrum for the binder.
Practically the X-ray diffraction technique allows to insure that the free magnesia is present in a lower proportion than 0.5% of the binder by dry weight.
Thus, the magnesia present in the raw material is almost combined into spinel. During the keramization step of a refractory concrete from the binder, since the hydraulic concrete dehydration may lead to a high steam pressure inside the concrete, crack generations due to the hydration of the magnesia into brucite are thus prevented.
By reference to UNITECR""97, the so-obtained concrete may further possess a particularly advantageously microstructure, since it comprises an intergranular matrix (between granulates of big size) formed with much finer grains. Such a property is due to the fact that the magnesian spinel in the binder can be easily milled and makes possible to produce very fine grains.
Preferably, the binder has the following chemical composition by dry weight of the binder:
More specifically, the binder has advantageously the following chemical composition, by dry weight of the binder:
The binder comprises advantageously a SiO2 content of less than 0.5% of the binder by dry weight.
Preferably, the binder has a Blaine area surface at least equal to 3000 cm2/g and advantageously higher than 4000 cm2/g.
This entity is measured according to the NF EN 196-6 standard. The binder comprises such characteristic after milling the clinker, the limit value indicated giving a preferred level of fineness of the grains which may be obtained with the binder according to the invention.
Another object of the invention is the use of a binder according to the invention for producing a refractory concrete.
Preferably, the binder is complemented by magnesian spinel, preferentially of large size, so that the concrete contains between 20% and 30% magnesian spinel by dry weight of the concrete.
Such spinel proportion is particularly advantageously, since it allows to obtain good resistances, both to corrosion and slag penetration.
More precisely the concrete is produced advantageously by mixing by dry weight of the binder:
between 16 and 27% of the binder,
between 2 and 13% of fine reactive alumina,
between 0 and 19% of large spinel, and
between 52 and 71% of alumina granulates.
In a particularly advantageous embodiment:
18% of the binder,
11% of fine reactive alumina,
11% of large spinel, and
60% of alumina granulates.
These proportions allow to produce a dense concrete with a theoretical compactness comprised between 0.25 and 0.40 because grain size lines are used that can suit to Andreasen mathematical model. The above-mentioned compositions authorize the proportion of 20% to 30% magnesian spinel.
In alternative embodiments, the reactive alumina being mixed with the binder is substituted by other materials.
The binder according to the invention is advantageously used in the manufacture of steel ladles, preferably for wear linings of such steel ladles.
The invention also relates to a process for producing a binder as defined above. According to the invention, the binder is made through frittering by burning of a blend of raw materials comprising dolomite, alumina and magnesia.
This blend, a source of CaO, MgO and Al2O3, has the advantage to give a very good sintering behaviour which can be appreciated by the quantity of uncombined magnesia staying after clinkerization.
Advantageously the raw materials have the following characteristics separately or in combination:
dolomite is natural: such dolomite leads, upon the decomposition thereof during the clinkerization, to the formation of very reactive products and has also the advantage to be economical;
alumina is metallurgical: such alumina has this advantage to be very reactive;
magnesia is reactive, preferably caustic, and has advantageously a grain size 100% lower than 100 xcexcm, preferably lower than 40 xcexcm: magnesia fine grain size is particularly interesting, since it favours a total combination of magnesia and consequently prevents residual magnesia to be present.
In two particularly advantageous embodiments, the following patterns are respectively used, where dolomite, alumina and magnesia are indicated by their tradenames:
Dolomite Saminxe2x80x94Alumina Sandyxe2x80x94Magnesia Briquette
Dolomite Saminxe2x80x94Alumina Pechineyxe2x80x94Magnesia MagChem40.
Preferably, before burning, the raw materials are milled up to a grain size corresponding to a 2% maximum rejection in a sieve of 65 xcexcm.
This co-milling of the raw materials allows to accelerate the chemical reactions in a solid phase.
Burning is advantageously carried out at a temperature comprised between 1400xc2x0C. and 1600xc2x0C.
Such relatively low burning temperatures are advantageous in the industrial and economical fields.
Advantageously, the degree of progression for burning is evaluated by measuring the free magnesia content by dry weight of the blend, for example by R-ray diffraction.
Such a content is in fact representative of the clinkerization being made.
When the clinker-type binder has been obtained, it is preferably milled. It is then advantageously used for making magnesian spinel based concrete.